At present fossil fuels are a dominant source of energy. The Hydrogen Economy is a proposed replacement for the current fossil fuel economy, in which renewably produced H2 is the primary energy carrier. Hydrogen has been shown to be a clean and renewable energy carrier with a high calorific value (H. Z. Wang et al, Renewal and Sustainable Energy Reviews, 2009, 13(4), p. 845-853). Although hydrogen is energy rich compared to petroleum on a per-weight basis, it is relatively poor on a volumetric basis (V. C. Y Kong et al, International Journal of Hydrogen Energy, 1999, 24(7) p. 665-6′75). Thus if portable hydrogen fuel cells are to be useful, then significant volumes of hydrogen will need to be carried “on-board”, unless high pressure or cryogenic hydrogen storage is used, both of which have significant energy penalties. To address this problem, the physical or chemical confinement of hydrogen, for example within carbon nanotubes (P. Bénard et al, Journal of Alloys and Compounds, 2007, 446-447, p. 380-384) or in metal hydrides (V. C. Y Kong et al, International Journal of Hydrogen Energy, 1999, 24(7) p. 665-675), has been the subject of a considerable body of research in materials chemistry. Guidelines set out by the US Department of Energy (DOE) target hydrogen storage materials with a hydrogen yield of 9.0 wt % and a specific energy of 10.8 MJ kg−1. However, the requirements that need to be met by the hydrogen storage material are very demanding; it must both readily absorb and release gas and be stable over many absorption-discharge cycles.
Some interest is thus now emerging in an alternative approach, which utilises the spontaneous reaction of a material (an “energy carrier”) with a liquid phase, typically an aqueous medium, to generate hydrogen at point of use. To produce a maximum yield on a weight-to-weight basis, light reactive elements are the most suitable. The theoretical yield of typical light and reactive metal and metal hydrides are compared in Table 1.
TABLE 1Theoretical hydrogen generation yield from reaction of light elementsand hydrides with water, calculated with the mass of hydrogenproduced normalised to the mass of material consumed.ElementLiNaMgAlSiMgH2NaAlH4wt % H214.54.48.311.214.3514.3214.93Specific energy20.76.2911.8716.0220.5220.4821.35(MJ Kg−1)
In theory, Si is perhaps the element which could best meet the various criteria (Auner, N. & Holl, S., 16th International Conference on Efficiency, Costs, Optimization, and Environmental Impact of Energy Systems 1395-1402, Pergamon-Elsevier Science Ltd, Copenhagen, DENMARK; 2003). The theoretical H2 yield from its reaction with water, as can be seen in Table 1, is higher than other rival elements. Indeed, silicon has a greater specific energy density than lighter elements such as Al, Mg, Na and Li, as the stoichiometry of the hydrolysis reaction is more favourable. Furthermore, it is an abundant element, comprising (as quartz sand) around 26% of the accessible earth's crust.
The feasibility of hydrogen generated from the reaction of pH neutral water and non-passivated silicon nano powder prepared by ball milling has been demonstrated in WO 2011/058317. Silicon can be prepared in a reactive form, using a simple process which reduces particle size and increases the reactive surface area of silicon without surface passivation. The resulting “nonpassivated silicon” can be reacted directly with water at low temperatures (<100° C.) to generate hydrogen in accordance with the following reaction:Si(s)+2H2O(l)→SiO2(s)+2H2(g)  (1)
The initial rate of hydrogen production observed in this reaction is typically from 20.4 to 55.8 cm3/min/g and conversion yield is typically from 36 to 67%.
Although this performance is superior compared to passivated silicon and non-passivated silicon from “dry” milling, there is still room to enhance both the kinetics of the hydrolysis reaction and the yield of hydrogen produced further to meet the requirement in practical applications.
There is, therefore, an ongoing need to provide improved materials and reaction systems which address these and other problems, and which can generate hydrogen at point of use towards a target of 9.0 wt % hydrogen in accordance with the DOE guidelines.